Nutrient cycling by large consumers at individual, population, and ecosystem levels

[English]
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Abstract

Organisms modulate nutrient cycles by transforming, storing, and transporting nutrients. While the impact of microorganisms and autotrophs on local and global biogeochemical cycles is well studied, our understanding of the nutrient cycling role of macro-consumers is in its infancy. In the following thesis, I explore aspects of the role of aquatic vertebrates in ecosystem nutrient cycling. Recent studies demonstrate substantial intraspecific variability in body element composition arising from environmental conditions and ontogeny. First, I test whether body element concentration varies among life stages and populations of Atlantic salmon (Salmo salar) from three Newfoundland rivers. I demonstrate that most intraspecific variability is explained by life stage and inter-stage variability in element concentrations can be attributed to the energy and nutrient requirements of reproduction and migration. Second, using long-term population monitoring data, I test whether ontogenetic differences in body phosphorus (P) concentration influence the role of Atlantic salmon as net sources or sinks of freshwater P. I find that incorporating inter-stage variability in body composition into nutrient flux models qualitatively changes our assessment of these populations as P sources or sinks relative to assuming ontogenetic homogeneity of body P concentration. Third, I develop a framework to describe the stoichiometric traits of vertebrate populations and use the framework to evaluate ontogenetic variability in body stoichiometry and total nutrient storage in brook trout (Salvelinus fontinalis) populations and partition nutrients released by migrating Atlantic salmon between eggs and excretion. Finally, life history strategy may influence interspecific variation in the ecosystem effects of migratory animals. I derive a two ecosystem model to investigate the ecosystem effects of migratory top consumers as subsidies. I formalized the hypothesis that iteroparous migratory animals should have stronger top-down effects on their biotic resource stocks than semelparous migratory animals, and that the response of ecosystem fluxes depends on the efficiency of consumer-mediated nutrient recycling. Overall, my findings suggest that interactions between ontogenetic development and life history strategy shape the nutrient cycling role of vertebrates. Connecting population structure and dynamics to nutrient cycles in this way may be a new path for 21st century ecological research and wildlife management.